Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) narrates: His paternal uncle struck a man from the polytheists and killed him, and because of that he himself was also wounded. He began to say: "I have killed myself!" News of this reached the Prophet (peace and blessings be upon him), and he said: "He will receive a double reward."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains narrators whom I do not know.